In wafer-to-wafer bonding technology, various methods have been developed to bond two package components (such as wafers) together. The available bonding methods include fusion bonding, eutectic bonding, direct metal bonding, hybrid bonding, and the like. In the fusion bonding, an oxide surface of a wafer is bonded to an oxide surface or a silicon surface of another wafer. In the eutectic bonding, two eutectic materials are placed together, and are applied with a specific pressure and temperature. In various conditions, the eutectic materials are melted. When the melted eutectic materials are solidified, the wafers are bonded together. In the direct metal-to-metal bonding, two metal pads are pressed against each other at an elevated temperature, and the inter-diffusion of the metal pads causes the bonding of the metal pads. In the hybrid bonding, the metal pads of two wafers are bonded to each other through direct metal-to-metal bonding, and an oxide surface of one of the two wafers is bonded to an oxide surface or a silicon surface of the other wafer.
The previously developed bonding methods have their disadvantages. For example, regarding the fusion bonding, extra electrical connections are needed to interconnect he bonded wafers. Accuracy of the eutectic bonding is low, and there may be metal-squeeze due to the melting of the bonding metals. Throughput of the direct metal-to-metal bonding is also low. In the hybrid bonding, the metal pads have higher Coefficients of Thermal Expansion (CTEs) than the dielectric layers at the surfaces of the bonded wafers. This results in problems in bonding the dielectric layers. For example, the bonds between the metal pads may delaminate if the expanded volume of the metal pads is smaller than the dishing volume of the metal pads. Conversely, if the expanded volume of the metal pads is significantly greater than the dishing volume, the bonds between the dielectric layers may delaminate.